I just moved into a new house and it's wired for ethernet. There's a junction box that has a "Bridged Telecom Module TM-8" in it. My cable modem's phone jack is plugged into the "Line In" on that box and their are 8 slots all running to various ports in the house. All the jacks in the house are RJ45. Is there a way to plug my old phones into these jacks or do I need to upgrade to phones that use this connection?

As emddudley said, it should plug right in. Just make sure you get the wiring correct. Plugging a phone into a jack wired for ethernet probably won't be a problem. Plugging a network device into a jack wired into the phone system is bad. Phones ring by having a voltage sent down the line, enough to ring a physical bell on old rotary phones. Network cards don't like getting this sent to them, and may release their magic smoke.